The goal of this project is to understand the molecular basis for the establishment of the dorsoventral pattern in the Drosophila compound eye. In each unit eye, the ommatidium, 8 photoreceptor cells are arranged in an asymmetric trapezoidal pattern. The dorsal eye field is in mirror symmetry with respect to the ventral field; the photoreceptor clusters are polarized in opposite directions across the equator, a midline that interfaces the dorso-ventral eye fields. The dorso-ventral mirror image is established as the photoreceptor clusters rotate 90 degrees clockwise or counter-clockwise, depending on the dorso-ventral polarity of each cluster. Only a few genes have been identified in the specification of these mechanisms. This study will use molecular and classical genetics to characterize an enhancer trap strain B1-12 that shows dorsal-specific white+ gene expression in the eye and two other genes involved in the rotation process, nemo and roulette. Systematic mutant screens will also be carried out to isolate additional mutants to dissect the genetic pathway for dorso-ventral pattern formation and the following rotation of photoreceptor clusters. The specific aims of this proposal are as follows: (1) We will isolate a candidate dorsal-specific gene B1-12, characterize eye phenotypes of B1-12 mutations produced by imprecise excision events, (2) We will identify additional genes involved in the dorso-ventral pattern by isolation of mutations that disrupt the dorsal-specific pattern of B1-12 and by enhancer trap screen for dorsal-, ventral-, or equator-specific expression of w+, (3) We will carry out a systematic genetic screen using the FRT/FLP recombination system to identify genes that control dorso-ventral polarity of photoreceptor clusters, (4) We will isolate roulette, a gene downstream of nemo in the rotation pathway, and (5) We will search for genetic modifiers of nemo mutation to identify interacting genes, and use the yeast two-hybrid system to isolate genes encoding interacting proteins. Bilateral symmetry and asymmetric polarity are common in the patterns of the body and its parts in most organisms. The problem of how the dorso- ventral polarity is established at the molecular and cellular levy is relevant to the phenomenon of situs inversus viscerum, the reversal of visceral asymmetry seen in mammals, which occurs as a result of abnormal determination of polarity. The basic mechanisms, and possibly the genes involved in tissue polarity may be structurally and functionally conserved between Drosophila and mammals. Given the powerful genetic methods available in Drosophila, it is proposed to use it as model organism to analyze these basic mechanisms.